Measurements and analysis of underwater acoustic noise of fishing vessels

Radiated underwater noise of a trawler has been measured with the purpose of investigating the characteristics of the radiated noise of fishing vessels in order to understand the relative large variations in catch rates of the vessels that are experienced. In addition to the standard analysis, the investigation includes studies of how the noise level varies with distance between vessel and hydrophone. The analysis software includes traditional Fourier analysis, combined with high-resolution spectral analysis algorithms and a special time-frequency analysis to detect and track narrow spectral lines. The measurements indicate that the noise propagation is dominated by the Lloyd mirror effect. By comparing, the observed sound level with the biologist´s estimate of the hearing and startling sensitivity the critical distance for scaring reduced catches is estimated.